1. Field of the Invention
This invention relates to metallic wire for electromagnetic applications such as electronic article surveillance (EAS); and more particularly to engineered glasses having in combination transverse geometric dimensions and mechanical properties that collectively impart superior mechanical integrity and magnetic properties to the glass-coated EAS wire.
2. Description of the Prior Art
Fine glass-coated metallic wire has been produced in a single step process without the use of quenching substrates.
A two-staged process as described by Taylor (Phys. Rev. 23, (1924) 655) involves drawing the end of a heated glass tube that contains molten alloy into a 0.5 to 1 mm diameter glass-coated rod having a length of a foot or more. This glass-coated rod is subsequently mechanically drawn through a heated die having holes the size of the final glass-coated wire diameter desired. Cooling rates employed by the Taylor method are much too low for production of metallic wire having an amorphous atomic structure.
U.S. Pat. No. 5,240,066 to Gorynin et al. discloses a method of casting amorphous and nanocrystalline alloy glass-coated wires. One of the problems with glass-coated wire produced by the '066 patent process is the tendency of glass to crack during manufacture or afterwards. The cracking problem is particularly acute when the product is exposed to changing temperatures. Another problem with the product produced by the '066 patent is its limited magnetic properties, for example low signal amplitude in the presence of an applied magnetic field. The restricted magnetic properties make the '066 product a poor candidate for use in EAS applications.
In U.S. Pat. No. 6,270,591 to Chiriac et al. and U.S. Patent Application Publication U.S. 2001/0001397 A1 (the '1397 application), there is disclosed a process as well as a glass-coated wire. The wire core is composed of an amorphous or nanocrystalline metallic alloy. Nowhere in either of these two references is a glass chemistry specified for preferential use in practice. In fact, only in Example 1 of each of these citations is reference made to the use of Pyrex®, which does not signify a specific glass chemistry, but rather a range of glass chemistries. For example, Corning Inc. offers two grades (7740 and 7789) of Pyrex for sale, one being slightly less dense and having slightly lower softening point than the other. Therefore, simply describing the use of Pyrex in glass-coated wire drawing operations is vague. As with the '066 patent, the glass-coated wire produced by Chiriac et al. and '1397 application exhibit limited magnetic response in the presence of an applied magnetic field. Such restricted magnetic properties impose reduced gating distance, limiting tag detection in EAS applications, and can trigger false alarms or fail to alarm during use of the tag in retail anti-theft systems. They also limit performance of the glass-coated wire product in other electromagnetic applications such as inductors, sensors, and transducers.